1. Field Of The Invention
This invention relates to the use of a Token Throwing Multiplexer (TTM) which enables users of two independent communication systems having multipoint networks and sited at the same location to share a single modem/telephone line multipoint network.
2. Description Of The Prior Art
A very common modern communication system is that commonly referred to as multipoint. In the multipoint system, multiple users share a common party line to a central site. The central site broadcasts to all users, each of which has a unique address. Based on the address, users act only upon those messages which contain their unique address. The communication line back to the central site is also shared. However, the communication systems protocol prohibits more than one user from transmitting at any given instant in time. This protocol is usually referred to as pole/response. FIG. 1 illustrates such a system wherein the communication lines are telephone lines and the communication devices are modems. The operation of such a system is explained in detail in U.S. Pat. No. 4,335,464, which has the same inventor and assignee as the present invention and which is incorporated herein by reference. Other references which are relevant to the aforementioned system are "Data Communications", Dixon R. Doll, John Wiley & Sons, New York, 1978; "Introduction to Teleprocessing", James Martin, Prentice Hall, Englewood Cliffs, N.J. 1972; "Data Communications", G. Hell and R. Sarch, McGraw-Hill Publications Co., N.Y. 1983; and "Systems Analysis for Data Transmission", James Martin, Prentice Hall, Englewood Cliffs, N.J., 1972.
As data communications requirements of businesses increase, many users have found the need for two or more multipoint networks at the same location as illustrated in FIG. 2. Such networks require two sets of modems and communication lines. The forementioned U.S. Pat. No. 4,335,464 provided a means whereby the two networks could be combined into one physical network of lines and modems and yet the system would appear to the user as two independent networks.
The aforementioned U.S. Pat. No. 4,335,464 permitted the two systems sharing the lines and modems to operate totally independently by using Frequency Division Multiplexing Techniques as shown in FIG. 4. This system is somewhat inefficient with regard to bandwidth because of the requirement for a guard band between the frequency multiplexed channels. Thus, FDM modems of the type in the aforementioned patent attain a bit rate of 9600 bits per second whereas more modern telephone line modems such as the Paradyne Corporation model VHS14.4 operate at 14,400 bits per second.